1. Field of the Invention
The present invention relates to a method for collecting 2,6-dichlorobenzonitrile obtained by ammoxidation of 2,6-dichlorotoluene and a process for purifying the same.
2. Prior Art
2,6-dichlorobenzonitrile is very important in industry not only as a herbicide but also as a starting material of insecticides in recent years. This compound is usually produced by ammoxidation process of 2,6-dichlorotoluene, that is, a process in which 2,6-dichlorotoluene is subjected to catalystic reaction with oxygen molecule in the presence of ammonia.
One of such processes is disclosed in U.S. Pat. No. 4,582,647.
That is, above-mentioned U.S. Patent discloses a process for producing pure dichlorobenzonitrile by ammoxidation of 2,6-dichlorotoluene using a catalyst containing vanadium-molybdenum oxide, which is characterized by conducting the reaction by a fluidized bed method and spraying water into the reaction gas effluent out of the fluidized bed reactor.
This process, however, lacks consideration on recovery and utilization of unreacted 2,6-dichlorotoluene containing in the reactor effluent gas.
By this process, a large amount of nitrogen gas such as 3.5 to 7.7 times (Examples 1 to 9) of air is added to the starting gases in the reactors (Therefore, the concentration of the 2,6-dichlorotoluene in the starting gases is 1.93 to 2.03 mole %). Addition of such a large amount of nitrogen is disadvantageous in not only increase of amount of heat required for preheating the starting gas and cooling the reactor effluent gas but also difficulty in recovery of the unreacted 2,6-dichlorotoluene by cooling the reaction gas. Also, this method is disadvantageous in a low yield such as 60 to 72% (in Examples 1 to 9 of above-mentioned U.S. Patent) of 2,6-dichlorobenzonitrile obtained.
Though this process seems advantageous because recrystallization from a solvent is not necessary, thus obtained "pure" 2,6-dichlorobenzonitrile contains the catalyst and/or heavy metallic compounds derived from materials of the apparatus very much, so it has problems in its use according to what the present inventors know. It is understood that the heavy metallic compounds derived from the materials of the apparatus are due to corrosion of the apparatus by ammonium chloride inevitably formed as a by-product because the ammoxidation of 2,6-dichlorotoluene is carried out at a relatively high temperature. Though the problems of these heavy metallic compounds may be solved by filtration of the solution of 2,6-dichlorobenzonitrile and recrystallization, this process even has problems in recovery of the solvent used.